The present invention relates to an optical integrated circuit which has branched optical waveguides, and more particularly to an optical integrated circuit which permits direct mounting thereon of a photodetector, a light emitting element, or similar optical active elements and can be miniaturized.
FIG. 1 shows a conventional optical integrated circuit, indicated generally by 28. On an electro-optic plate 11 as of lithium niobate (LiNbO.sub.3) there is formed an optical waveguide 12, which has one end branched into optical waveguides 13 and 14 extending to optical waveguide terminals 15 and 16 at one side of the electro-optic plate 11 and has its other end branched into optical waveguides 17 and 18 extending to optical waveguide terminals 21 and 22 at the opposite side of the electro-optic plate 11. Electrodes 23 are formed on both sides of the parallel-extending portions of the optical waveguides 17 and 18 near the terminals 21 and 22 to provide an optical phase modulating function.
For input and output of light with respect to such an optical integrated circuit, it is a common practice in the prior art to employ a method in which an optical fiber has one end face fixedly bonded to each optical waveguide terminal, or a method in which optical fibers of the same number as that of the optical waveguide terminals at one side of the electro-optic plate 11 are fixed to an optical fiber holder which is connected to the optical integrated circuit.
FIGS. 2A and 2B are a front view and a plan view of an optical fiber holder for holding two optical fibers. The optical fiber holder is composed of two holder halves 24 and 25, which are assembled together to fixedly hold two optical fibers 26 and 27 at one end disposed in parallel as shown and their end faces are ground or polished. The core spacing D.sub.2 of the optical fibers 26 and 27 is equal to the outer diameter of each optical fiber. Since the outer diameter of the optical fiber is 125 .mu.m, for example, in the case of a single mode optical fiber commercially available in Japan, the spacing D.sub.2 is 125 .mu.m, for instance. Accordingly, the distance D.sub.1 (FIG. 1) between optical waveguide terminals at one side of the optical integrated circuit 28 is also designed to be 125 .mu.m.
The angle of bend .theta. at which the optical waveguide is branched is several degrees or less in many cases. Now, consider that the angle of bend .theta. of the branched optical waveguide 17 to the optical waveguide 12 is, for example, 1.degree. in FIG. 1. In order that the spacing D.sub.1 may be 125 .mu.m, the lengths L.sub.2 of those portions of the branched optical waveguides 17 and 18 which are inclined to the optical waveguide 12 are about 3.6 mm. In the case where the length L.sub.3 of the optical waveguide 12 is 7 mm, the length L.sub.5 of each of the parallel portions of the optical waveguides 17 and 18 where the optical phase modulating function is provided is 12 mm and the length L.sub.1 of each of the parallel portions of the optical waveguides 13 and 14 is 3 mm, the entire length L of the optical integrated circuit is 29.2 mm.
The optical integrated circuit shown in FIG. 1 is used in a fiber optic gyro, for instance. FIGS. 3 and 4 show an example of the fiber optic gyro, in which a bobbin 32 with an optical fiber coil 33 wound thereon is mounted on a chassis 31 covered with a cover 29, a light source module 34 and a photodetector module 35 are disposed inside the bobbin 32, a base plate 36 is disposed on an upper plate 32A of the bobbin 32 and the optical integrated circuit 28 is mounted on the base plate 36 through a mounting plate 37.
Optical fiber holders 39 and 41, each holding two optical fibers, are fixedly bonded to the two optical waveguide terminals at the opposite sides of the optical integrated circuit 28. The one optical fiber 42 held by the holder 39 is connected by fusion to an optical fiber 43 extending from the light source module 34, whereas the other optical fiber 44 is connected by fusion to an optical fiber 45 extending from the photodetector module 35. Optical fibers 46 and 47 held by the holder 41 are each connected by fusion to one end of the optical fiber coil 33. Conventionally, the optical integrated circuit and other modules are thus connected using optical fibers.
In the optical integrated circuit 28 depicted in FIG. 1, since the distance D.sub.1 between two optical waveguide terminals is only 125 .mu.m or so, a photodetector and a light emitting element, or an optical fiber holder and a photodetector or light emitting element, each having a 1 mm or more chip size, cannot be mounted side by side on the optical integrated circuit 28. To mount the photodetector and the light emitting element side by side on the optical integrated circuit, it is necessary to increase the distance D.sub.1 between the optical waveguide terminals, and this calls for an increase in the length L.sub.2 of each of the inclined portions of the branched optical waveguides 13 and 14, inevitably making the optical integrated circuit bulky.